The present invention relates generally to power transmission systems and more particularly to providing superconducting magnetic energy storage for two or more ac power transmission systems.
An asynchronous tie (AST) is an electrical connection in the form of a high voltage direct current (HVDC) link used for power transmission between two ac systems that differ at least in one of the three characteristic parameters of an ac system: amplitude, frequency or phase. Such ac systems cannot be connected directly. An AST generally consists of two ac/dc converters connected by two conductors. Power of the first ac system is rectified in the first converter, inverted in the second converter and consumed in the second ac system. By proper control of both converters, the energy flow between both ac systems is bi-directional. Because no energy storage is involved in AST, energy sent into one ac system is absorbed at the same rate in the second ac system for assumed lossless power transmission in the AST. Several ASTs are presently operating or being installed in the United States, see Hingoani, EPRI Journal, pg. 16 (January-February 1983).
A superconducting magnetic energy storage (SMES) device is capable of storing electrical energy in a magnet during a certain time period and then releasing the energy back into the electrical system over a different time period. The economic incentive for such a device is storing low cost, off-peak power and releasing that energy as peak power. In a typical SMES system, a superconducting magnet is interfaced to an ac system by an ac/dc converter. Energy from the ac system is absorbed in the superconducting coil and released back to the same ac system. The converter determines the role and direction of the energy flow. The principle of operation of a SMES system and its beneficial affect on an electric utility system has been shown, see U.S. Pat. No. 4,122,512. The first large scale SMES unit in the world was recently installed by the Los Alamos National Laboratory in Tacoma, Wash., see Rogers, IEEE Proceedings (1983).
It is an object of the present invention to provide superconducting magnetic energy storage for a plurality of asynchronous electrical systems.
It is a further object of the present invention to provide load leveling and stability improvement in a plurality of independent ac systems using a single superconducting magnetic energy storage coil.